Optical leaky integrate-and-fire neuron

ABSTRACT

An optical system includes an optical integrator, a readout mechanism, and an optical thresholder. The optical integrator is configured to perform temporal integration of an optical input signal having a first wavelength received at an input. The readout mechanism is coupled to the optical integrator and provides optical signals having a second wavelength to the optical integrator for measuring a state of the optical integrator. The optical thresholder is coupled to an output of the optical integrator and is configured to receive a signal representing a temporal integration of the optical input signal from the optical integrator and produce an optical signal identifying if an amplitude of the signal representing the temporal integration of the optical input signal is above or below a threshold value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 61/158,986, which was filed on Mar. 10, 2009, and is incorporated by reference herein in its entirety.

FIELD OF DISCLOSURE

The disclosed circuit and method relate to optical circuits. More specifically, the disclosed circuit and method relate to an all-optical circuit for performing computations.

BACKGROUND

Signal processing continues to become more complex as data transfer rates continue to increase. Conventionally, signal processing is either performed by an analog system or by a digital system. Analog systems may be implemented in compact circuits making them a popular choice when circuit area is of significant importance. However, one significant drawback of analog devices is that they are highly susceptible to noise accumulation, which limits the number of analog operations that can be applied to data, and therefore the complexity of the computations that can be practically implemented using only analog devices.

In contrast, digital systems are not as susceptible to noise accumulation as are analog systems. However, the number of digital devices needed to implement a computation rapidly increases with the complexity of the computation performed.

Photonic devices provide the ability to process signals of much higher bandwidth than is possible with electronic devices, but they are larger and more expensive than electronic devices. Practical implementation of complex high bandwidth processing algorithms using photonic devices therefore requires an approach that minimizes the number of devices needed without over constraining the complexity of computations that can be implemented.

Accordingly, a hybrid processing system and method that combines the advantages of digital and analog systems is desirable.

SUMMARY

An optical system is disclosed that includes an optical integrator, a readout mechanism, and an optical thresholder. The optical integrator is configured to perform temporal integration of an optical input signal having a first wavelength received at an input. The readout mechanism is coupled to the optical integrator and provides optical signals having a second wavelength to the optical integrator for measuring a state of the optical integrator. The optical thresholder is coupled to an output of the optical integrator and is configured to receive a signal representing a temporal integration of the optical input signal from the optical integrator and produce an optical signal identifying if an amplitude of the signal representing the temporal integration of the optical input signal is above or below a threshold value.

A signal processing method is also disclosed. The optical signal processing method includes temporally integrating a first optical signal having a first wavelength at an optical integrator, determining if the temporally integrated optical signal has an amplitude that is above a threshold at an optical thresholder, and outputting an optical signal identifying if the amplitude of the temporally integrated optical signal is above or below the threshold.

Additionally, an optical system is disclosed including a semiconductor optical amplifier (SOA), an optical filter, and an optical thresholder. The SOA has a decaying response function for temporally integrating an optical signal having a first wavelength received at an input. The SOA is configured to output an optical signal identifying a state of the SOA in response to receiving a signal having a second wavelength from a readout device. The optical filter is coupled to an output of the SOA and is configured to pass the optical signals having the second wavelength and blocking optical signals having the first wavelength. The optical thresholder is configured to receive optical signals having the second wavelength identifying the state of the SOA from the optical filter and provide an optical signal identifying if an energy of the optical signals having the second wavelength are above or below a threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of one example of an optical neuron.

FIG. 1B is a block diagram illustrating the integration and thresholder blocks in accordance with the optical neuron illustrated in FIG. 1A.

FIG. 2 is a block diagram of one example of a semiconductor optical amplifier.

FIG. 3A illustrates one example of input pulses received at an input of the optical integrator.

FIG. 3B illustrates one example of output pulses from the optical integrator in response to the receiving the input pulses illustrated in FIG. 3A.

FIG. 4A is an energy versus time graph showing the response of a semiconductor optical amplifier that receives a series of optical pulses.

FIG. 4B is an example oscilloscope trace of a plurality of optical signals of a pulse train.

FIG. 5A illustrates one example of output pulses from an optical thresholder having a threshold set between zero and one in response to receiving the pulses illustrated in FIG. 3B.

FIG. 5B illustrates one example of output pulses from an optical thresholder having a threshold set between one and two in response to receiving the pulses illustrated in FIG. 3B.

DETAILED DESCRIPTION

The leaky integrate-and-fire (“LIF”) neuron is one of the most widely studied neuron models in computational neuroscience. The spike processing performed by these computational elements is a hybrid of analog and digital processing that exploits the efficiency of analog computation while overcoming the problem of noise accumulation suffered by analog systems. In spike processing, information is encoded in the timing of spikes rather than in the size or shape of the spikes. Accordingly, information is conveyed by a spike being present or absent, much like in digital systems how a bit is either a one or a zero. In contrast, traditional neural network models perform purely analog computation in which neuron inputs, intermediate results, and outputs are represented as analog values.

From the standpoint of computability and complexity theory, LIF neurons are powerful computational primitives capable of simulating both Turing machines and traditional neural networks. LIF models have a number, N, of inputs, σ_(i)(t), where i=1,2, . . . , N; an internal activation state, V_(m)(t); and a single output state, O(t). At rest, the internal state of the neuron is actively maintained at a resting voltage, V_(rest). Each input, σ_(t)(t), of the neuron is a continuous time series consisting of either spikes or continuous analog values. These inputs are typically weighted by ω_(i) and delayed δ_(i), which may be mathematically represented as ω_(i)σ_(i)(t+δ_(i)). The delayed and weighted input time series is spatially integrated through pointwise summation in accordance with the following equation:

Σ_(i=1) ^(N)ω_(i)σ_(i)(t+δ _(i))  Eq. (1)

The activation state, or membrane voltage, V_(m)(t), of the neuron is an exponentially weighted temporal integration of the spatially summed input time series. If the magnitude of the temporally integrated signal exceeds a threshold value, then the neuron outputs a spike, e.g., O(t)=1, if V_(m)(t)>V_(thresh). After a spike, there is a short period of time, known as a refractory period, during which another spike cannot be issued, e.g., if O(t)=1, then O(t+Δt)=0, Δt≦T_(refract). Accordingly, the output of the neuron, O(t), consists of a continuous time series of spikes.

There are three primary influences that affect the magnitude of the membrane voltage, V_(m)(t), of an LIF neuron: (1) an active pumping current, (2) current leakage, and (3) external inputs generating time varying membrane conductance changes. Each of these influences are part of the following differential equation for approximating the membrane voltage over time:

$\begin{matrix} {\frac{{V_{m}(t)}}{t} = {\frac{V_{rest}}{\tau_{m}} - \frac{V_{m}(t)}{\tau_{m}} + {\frac{1}{c_{m}}{V_{m}(t)}{\sigma (t)}}}} & {{Eq}.\mspace{14mu} (2)} \end{matrix}$

Where,

$\frac{{V_{m}(t)}}{t}$

represents the activation of the neuron;

$\frac{V_{rest}}{\tau_{m}}$

represents the active pumping current of the neuron;

$\frac{V_{m}(t)}{\tau_{m}}$

represents the leakage current of neuron; and

$\frac{1}{c_{m}}{V_{m}(t)}{\sigma (t)}$

represents the external inputs to the neuron.

A direct correspondence has been discovered between the equation governing temporal integration of LIF neurons set forth in Equation 2 and the carrier density of a semiconductor optical amplifier (SOA). The primary state variable for a SOA in this case is the carrier density above transparency N′(t)=N(t)−N₀, where N(t) is the actual carrier density, and N₀(t) is carrier density at transparency. The integrative properties of the SOA are determined by the carrier lifetime, τ_(e); a mode confinement factor, ┌; a differential gain coefficient, α; a photon energy, Ep; and the active SOA pumping current, I(t). Spontaneous carrier decay tends to drive the carrier density, N′, of the SOA towards zero, and thus the active pumping current is needed to counter the carrier decay to maintain a resting carrier density of N′_(rest). The three contributions to the value of the carrier density, N′(t), are a leakage term due to passive carrier decay, a term for carrier density due to active optical pumping of the SOA, and a term for carriers generated by external inputs. The gain dynamics of a SOA when input pulse widths are much shorter, e.g., two orders of magnitude shorter, than the carrier lifetime may be described as follows:

$\begin{matrix} {\frac{{N^{\prime}(t)}}{t} = {\frac{N_{rest}^{\prime}}{\tau_{e}} - \frac{N^{\prime}(t)}{\tau_{e}} + {\frac{\Gamma_{a}}{E_{p}}{N^{\prime}(t)}{I(t)}}}} & {{Eq}.\mspace{14mu} (3)} \end{matrix}$

Comparing Equation 2 with Equation 3 demonstrates a remarkable similarity between the electrical model of membrane voltage of an LIF neuron and the optical model of SOA carrier density. The discovery of the correspondence between the Equations 1 and 2 has enabled the development of an all-optical implementation of an LIF neuron, which can advantageously be used as a computational primitive in large scale complex photonic computational systems.

FIG. 1A is a block diagram of one example of an optical implementation of an LIF neuron 100. As shown in FIG. 1A, the optical LIF neuron 100 includes an optical coupler 106-1 configured to receive N optical input signals. Each input to the optical coupler 106-1 may include a variable attenuator 102-1:102-N and a variable delay line 104-1:104-N. Accordingly, the variable attenuators 102-1:102-N, variable delay lines 104-1:104-N, and optical coupler 106-1 respectively provide the passive weighting, delay, and the summation of inputs of the optical LIF neuron 100. The optical coupler 106-1 is coupled to an integrator block 108 which is also coupled to a readout mechanism 112 for reading out the state of the integrator block 108. The state of integrator block 108 is output to a thresholder block 110 in the form of an optical signal. Auxiliary devices such as supplementary electronics and optical modulators may be included, but are not shown to simplify the figure.

FIG. 1B is a more detailed schematic of the optical integrator and thresholder 108, 110. As shown in FIG. 1B, the optical integrator 108 includes a second optical coupler 106-2 having an output coupled to an input of SOA 114. The SOA 114 also has an output coupled to an input of optical filter 116, and a charge pumping circuit 118 is provided for pumping the SOA 114 with electrons. An optional optical amplifier 120 is shown coupled between an output of the integrator 108 (e.g., an output of the optical filter 116) and an input of the thresholder 110.

The thresholder 110 includes a non-linear optical loop mirror 122 formed by a non-linear doped fiber 124 and an optical coupler 126. A plurality of optical components may be disposed along non-linear fiber 124 for optical tuning. For example, polarization controllers 128-1, 128-2 and a tunable isolator 130 may be disposed along the non-linear fiber 124.

Optical couplers 106 (e.g., optical couplers 106-1 and 106-2) may be any optical coupler configured to couple optical signals of different wavelengths and amplitudes in separate fibers into a single fiber. In one example, the optical coupler 106-1 is an N:1 optical coupler configured to couple N optical input signals into a single fiber, and optical coupler 106-2 is an optical coupler configured to couple the optical input signals output from optical coupler 106-1 with optical signals from a pulse train provided by the optical readout mechanism 112 into a single fiber. An example of a suitable fiber coupler 106-2 is a thermally tapered and fused pair of single-mode fibers, with the cores of the fiber pair coming into contact such that optical energy may be exchanged. Multiport coupler 106-1 may be, for example, a tree of 2:1 couplers as will be understood by one skilled in the art. The optical signals of the sampling pulse train may have a wavelength λ₀, and the optical input signals may have one or more wavelengths λ₁, λ₂, etc., which are different from the wavelength of the pulse train. Additionally, the optical input signals have amplitudes that are greater than the amplitudes of the optical signals of the pulse train such that the optical signals of the pulse train do not have a significant effect on the cross-gain modulation (XGM) of the SOA 114 as described below. For example, the amplitudes of the optical input signals may be ten times the amplitude of the optical signals of the pulse train. However, one skilled in the art will understand that the difference between the amplitudes of the optical input pulses and the optical signals of the pulse train may be increased or decreased.

Readout mechanism 112 (FIG. 1A) may be any device configured to provide a signal for reading out a current state of SOA 114. For example, readout mechanism 112 may be an optical pulse train generator for providing optical signals having uniform wavelengths and amplitudes such as, for example, a mode-locked ring fiber laser (MLL) configured to provide pulses on the order of picoseconds.

One example of an SOA 114 is illustrated in FIG. 2. As shown in FIG. 2, the SOA 114 includes a semiconductor substrate 200, which may be a Group III-V compound substrate as will be understood by one skilled in the art. Substrate 200 may be an n-type substrate having an n-doped region 202 and a p-doped region 204. Metal layers 206 and 208 may be formed on a top and a bottom surface of the substrate 200. As shown in FIGS. 1B and 2, the charge pumping circuit 118 is coupled to SOA 114 and configured to restore the gain of SOA 114 through population inversion after the gain of the SOA 114 has been depleted. The charge pumping circuit 118 may be implemented as an electrical circuit in which a current is supplied to the substrate of the SOA 114, or the charge pumping circuit 118 may be implemented as an optical circuit in which light is used to perform population inversion of the SOA 114.

Referring again to FIG. 1B, optical filter 116 may be a short-pass, a long-pass, or a band-pass optical filter configured to block the wavelengths of the optical input signals and pass optical signals provided by the readout mechanism 112. Examples of optical filter 116 include, but are not limited to, thin film multi-layer dielectric filters, Bragg gratings, and arrayed waveguide gratings.

The optical amplifier 120 may be any optical amplifier configured to increase the amplitude of an optical signal. In one arrangement, the optical amplifier 120 is an erbium-doped fiber amplifier (EDFA). Another example of optical amplifier 120 is an erbium-ytterbium-doped fiber amplifier, which may provide a higher output power than an EDFA.

The non-linear fiber 124 of thresholder 110 may be a GeO₂-doped silica-based non-linear fiber. Other non-linear fibers such as, for example, microstructured fibers, photonic crystal fibers, and hi-oxide fibers, to name a few, may also be implemented. Parasitic reflections may be suppressed for proper thresholder operation as will be understood by one skilled in the art.

Polarization controllers 128-1, 128-2 may use a controllable bend of fiber to control the polarization of light and may be based on a medium having a weak controllable birefringence. Tunable isolator 130 may be any kind of optical isolator and may be based on the Faraday effect and have a controllable leak in a backward direction. Coupler 126 may be the same type of optical coupler as couplers 106-1, 106-2 except that it may have an unequal coupling ratio. Examples of coupling ratios include, but are not limited to, an 80:20 ratio, a 90:10 ratio, to name a few.

An optional optical inverter 132 may be coupled to the output of the thresholder 110 for inverting the thresholder output. In some embodiments, the optical inverter 132 may be an optical logic gate such as the one taught by Miyoshi et al. in Ultrafast All-Optical Logic Gate Using a Nonlinear Optical Loop Mirror Based Multi-Periodic Transfer Function, Optics Express, Vol. 16, Issue 4, 2570-2577 (2008), the entirety of which is incorporated by reference herein. The optical logic gate may be configured to output an optical signal having the same characteristics of the optical input signals (e.g., wavelength and amplitude) in response to the output of the thresholder 110. One skilled in the art will understand that other optical devices may be used to invert the optical signal output from the thresholder 110. For example, the optical inverter 132 may include an SOA for inverting the optical signal or an optical data format converter based on a tetrahertz optical asymmetrical demultiplexer (TOAD) such as the one disclosed in U.S. Pat. No. 6,448,913 issued to Prucnal at al., the entirety of which is incorporated by reference herein.

With reference to FIGS. 1A and 1B, the operation of the LIF optical neuron 100 is now described. One or more optical input signals having a wavelength λ₁ are coupled together at the first optical coupler 106-1. The coupled optical input signals having one or more wavelengths λ₁, λ₂, etc. are coupled with optical signals of the pulse train having a different wavelength λ₀ provided by the readout mechanism 112 at the second optical coupler 106-2. As described above, the amplitude of the pulsed optical input signals may be ten time greater than the amplitude of the optical signals of the pulse train.

Optical coupler 106-2 outputs a multiple wavelength optical signal to an input of the SOA 114. SOA 114 of the integration block 108 is pumped with electrons from a charge pump circuit 118, which performs population inversion of the SOA 114. When a pulse from one of the optical input signals is received at the SOA 114, the gain of the SOA 114 is depleted due to the depletion of charge that occurs due to XGM. The external pumping of the SOA 114 causes the gain of the SOA 114 to gradually increase, but if another pulse is received from the optical input signals, then the gain of the SOA 114 will again be depleted. The recovery time of the gain of the SOA 114 is based on the carrier lifetime, T_(e), which functions as the integration time constant of the integrator 108. Thus, the smaller the carrier lifetime of the SOA 114 the faster the gain of the SOA 114 recovers and less temporal integration of the input signals is performed.

The integrated optical signal is output from the SOA 114 and received at the optical filter 116. As described above, the optical filter 116 may be tuned such that the optical filter 116 passes the optical signals having a wavelength λ₀ and the optical input signals are blocked, reflected, or otherwise filtered out.

The filtered optical signal is received at an input of the optical amplifier 120, which may be an EDFA as described above. The optical amplifier increases the amplitude of the filtered and integrated optical signal and outputs the amplified optical signal to the thresholder 110. The gain of the optical amplifier 120 may be designed to provide sufficient amplification of the filtered optical signal so that the amplitude of the signal falls within the linear region of the non-linear fiber 124 of the thresholder 110.

Thresholder 110 outputs a signal identifying if the optical signal received at the input is greater than or equal to a predetermined threshold level. However, due to the utilization of the SOA 114 with gain sampling, the output of the thresholder when one of the optical input signals is a logic one, the thresholder will output a logic zero.

As described above, an optical inverter 132 may be coupled to the output of the thresholder 110 to invert the signal output from the thresholder 110. For example, if the inverter 132 is an optical logic gate such as the ones described by Miyoshi et al., the inverter may output an optical signal having a wavelength of λ₁ and an amplitude equal to the amplitude of the optical input signal when the thresholder 110 outputs a logic zero. The output of the optical inverter 132 may be fed into another processing element as will be understood by one skilled in the art. For example, the output of optical inverter 132 may be fed into another thresholder to provide standardization of the pulses at the neuron output according to the LIF neuron model.

An optical LIF neuron 100 as illustrated in FIGS. 1A and 18 and described above was designed and tested. The optical LIF neuron was implemented with the N:1 optical coupler 106-1 having two inputs, i.e., N=2. A supercontinuum generator with spectral slicing was utilized to provide the optical signals for the pulse trains for multiple wavelengths. The resulting width of optical signals of the pulse train was about 3 ps at full-width half maximum (FWHM). Mach-Zehnder optical modulators and a standard bit-error tester were used for creating different pulse patterns required in performed measurements. A master pulse source having a 1.25 GHz mode-locked ring fiber laser was used to generate the optical input signals. Five-bit patterns of ‘01100’ with a delay of approximately 1 bit were input to each of the inputs of the optical coupler 106-1 for a net input of ‘01210’ into the optical system 100. The optical inputs were coupled with the optical signals of the pulse train provided by the 1.25 GHz mode-locked ring fiber laser at an optical coupler 116-2 to provide an input to the SOA 114 as illustrated in FIG. 3A. SOA 114 was an Alcatel A1901SOA available from Alcatel-Lucent of Murray Hill, N.J.

The gain dynamics of the tested SOA 114 are shown in FIG. 4A. The y-axis in FIG. 4A corresponds to the energy of the sampling pulses, which is proportional to the SOA gain, N′(t). The “resting potential”, i.e. the maximum SOA gain when no control signal is present, was approximately equal to 43 fj. FIG. 4B illustrates an oscillogram of the control signal in the same time scale as the time scale of FIG. 4A. As shown in FIGS. 4A and 4B, each pulse of the optical input signal leads to a decrease of energy of sampling pulses due to the XGM in the SOA 114. After a pulse depletes the gain of the SOA 114, the power gradually increases while SOA gain recovers. The input pulses create a saw-tooth SOA gain curve as shown in FIG. 4A. The integration time constant, i.e. the SOA carrier lifetime, T_(e), was able to be adjusted in the range of approximately 100 to 300 ps by changing the SOA pump current supplied by the pump circuit 118 from approximately 70 mA to approximately 170 mA. In the example illustrated in FIGS. 4A and 4B, the carrier lifetime, T_(e), was approximately equal to 180 ps.

The output of the integrator 108 is illustrated in FIG. 3B. As shown in FIG. 3B, the integrator block 108 outputs optical pulses at three different levels, e.g., zero, one, and two, which are the inverse of the received pulses. Accordingly, when both optical input signals were logic ones, then the sum of the inputs was a two, which the integration block 108 output as a zero.

The thresholder 110 was constructed using a non-linear fiber based on a modified nonlinear optical loop mirror. The thresholder 110 included 10.5 m of a silica-based non-linear fiber 124 heavily-doped with GeO₂ (preform 311). The fiber parameters measured at λ=1550 nm were a nonlinear coefficient 35 W⁻¹ km⁻¹; propagation losses of 36 dB/km; chromatic dispersion −70 ps/nm km; and a refraction index difference, Δn, of approximately 0.11. The idealized model of the thresholder 110 predicted that the output power was proportional to the cube of the input power, A measured transfer function had a cubic dependence for some range of input powers with saturation at a certain input level at which the nonlinear phase shift approaches π. The input power of the thresholder was controlled by adjusting the gain of the optical amplifier 120, which was implemented as an EDFA with a maximum output power of approximately 23 dBm.

The neuron 100 was able to discriminate between the lowest and middle pulse energies, e.g., a zero or a one, or between the middle and the highest pulse energies, e.g., a one and a two. Both possibilities were experimentally demonstrated in the setup with corresponding diagrams shown in FIGS. 5A and 5B. Also, two cases of thresholding with the threshold below the smallest pulse energy and above the highest were also realized, but are not shown. Accordingly, the thresholder was capable of clear separation between a pulse and no pulse at its input depending on the threshold.

The experimentally demonstrated photonic LIF device was shown to be operable using picosecond-width pulses and have an integration time constant of 180 ps, which was adjustable within the range of approximately 100 to 300 ps. Reconfiguration of device parameters enables it to perform a wide variety of signal processing and decision operations, its analog properties makes it well-suited for efficient signal processing applications. The digital properties of the optical LIF neuron make it possible to implement complex computations without excessive noise accumulation.

Although the systems and methods have been described in terms of exemplary embodiments, they are not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the systems and methods, which may be made by those skilled in the art without departing from the scope and range of equivalents of the systems and methods. 

1. An optical system, comprising: an optical integrator for performing temporal integration of an optical input signal having a first wavelength received at an input; a readout mechanism coupled to the optical integrator for providing optical signals having a second wavelength to the optical integrator for measuring a state of the optical integrator; and an optical thresholder coupled to an output of the optical integrator for receiving a signal representing a temporal integration of the optical input signal from the optical integrator and producing an output identifying if an amplitude of the signal representing the temporal integration of the optical input signal is above or below a threshold value.
 2. The optical system of claim 1, wherein the optical integrator includes: a semiconductor optical amplifier (SOA) having a decaying response function, the SOA configured to receive the optical signals having the first and second wavelengths and first and second amplitudes and provide the signal representing temporal integration of the optical input signal in response, and an optical filter coupled to an output of the SOA, the optical filter for passing optical signals having the second wavelength and blocking the optical signals having the first wavelength.
 3. The optical system of claim 1, wherein the optical thresholder includes a first polarization controller and a non-linear fiber optical loop mirror.
 4. The optical system of claim 1, further comprising: an optical amplifier disposed between an output of the optical integrator and an input of the optical thresholder for amplifying the signal representing the temporal integration of the optical input signal output from the optical integrator.
 5. The optical system of claim 1, wherein the readout mechanism is an optical pulse train generator providing an optical signal having a lower energy than an energy of the optical input signal.
 6. The optical system of claim 1, further comprising: a first optical coupler having first and second inputs, the first optical coupler for coupling a first optical input signal having the first wavelength with a second optical signal having the first wavelength into a single fiber to provide the optical input signal, the first input of the first optical coupler for receiving the first optical signal having the first wavelength, the second input of the first optical coupler for receiving the second optical signal having the second wavelength; and a second optical coupler having third and fourth inputs and an output, the third input coupled to an output of the first optical coupler, the fourth input for receiving the optical signals having the second wavelength, the second optical coupler for coupling the optical input signal having the first wavelength with the optical signals having the second wavelength into a single fiber coupled to an input of the optical integrator.
 7. The optical system of claim 6, further comprising: a first variable attenuator for providing a weight to an optical signal; and a first variable delay line for delaying an optical signal coupled to the first variable attenuator, wherein one of the first variable attenuator and the first variable delay line is coupled to the first input of the first optical coupler.
 8. The optical system of claim 7, further comprising: a second variable attenuator; and a second variable delay line coupled to the second variable attenuator, wherein one of the second variable attenuator and the second variable delay line is coupled to the second input of the first optical coupler.
 9. An optical signal processing method, comprising: temporally integrating a first optical signal having a first wavelength at an optical integrator; determining if the temporally integrated optical signal has an amplitude that is above a threshold at an optical thresholder; and outputting an optical signal identifying if the amplitude of the temporally integrated optical signal is above or below the threshold.
 10. The method of claim 9, further comprising: combining the first optical signal having the first wavelength with an optical signal having a second wavelength into a single fiber at a first optical coupler, the first optical coupler having an output coupled to an input of the optical integrator.
 11. The method of claim 10, further comprising: combining a second optical signal having the first wavelength with a third optical signal having the first wavelength at second optical coupler to provide the first optical signal having the first wavelength; and outputting the first optical signal having the first wavelength to the first optical coupler.
 12. The method of claim 9, further comprising: increasing an amplitude of the temporally integrated optical signal at an optical amplifier prior to determining if the amplitude of the temporally integrated optical signal is above the threshold level.
 13. The method of claim 9, wherein the thresholder includes a non-linear optical loop mirror and a tunable optical isolator.
 14. The method of claim 11, further comprising: adding a weight to at least one of the first and second optical signals having the first wavelength prior to combining them.
 15. The method of claim 11, further comprising: delaying at least one of the first and second optical signals having the first wavelength using a variable delay line prior to combining them.
 16. An optical system, comprising: a semiconductor optical amplifier(SOA) having a decaying response function for temporally integrating an optical signal having a first wavelength received at an input, the SOA configured to output an optical signal identifying a state of the SOA response to receiving a signal having a second wavelength from a readout device; an optical filter coupled to an output of the SOA, the optical filter for passing the optical signals having the second wavelength and blocking optical signals having the first wavelength; and an optical thresholder for receiving optical signals having the second wavelength identifying the state of the SOA from the optical filter and providing an optical signal identifying if an energy of the optical signals having the second wavelength are above or below a threshold value.
 17. The optical system of claim 16, further comprising: an optical amplifier disposed between the output of the optical filter and the input of the optical thresholder, the optical amplifier for amplifying the optical signals having the second wavelength received from the optical filter.
 18. The optical system of claim 16, wherein the optical thresholder includes a non-linear optical loop mirror and a tunable optical isolator.
 19. The optical system of claim 16, wherein an amplitude of the optical signal having the second wavelength is smaller than an amplitude of the optical signal having the first wavelength.
 20. The optical system of claim 16, further comprising: a first optical coupler having first and second inputs for summing the first optical signal having the first wavelength with a second optical signal having the first wavelength, the first input of the first optical coupler for receiving the first optical signal having the first wavelength, the second input of the first optical coupler for receiving the second optical signal having the first wavelength; and a second optical coupler having third and fourth inputs and an output, the third input coupled to an output of the first optical coupler, the fourth input for receiving the signal having the second wavelength, the output of the second optical coupler coupled to an input of the SOA.
 21. The optical system of claim 20, further comprising: a first variable attenuator for providing a weight to an optical signal; and a first variable delay line for delaying an optical signal coupled to the first variable attenuator, wherein one of the first variable attenuator and the first variable delay line is coupled to the first input of the first optical coupler.
 22. The optical system of claim 21, further comprising: a second variable attenuator for delaying an optical signal; and a second variable delay line for delaying an optical signal coupled to the second variable attenuator, wherein one of the second variable attenuator and the second variable delay line is coupled to the second input of the first optical coupler. 